The present invention pertains to the field of flat panel display screens. More specifically, the present invention relates to the field of brightness corrections for flat panel field emission display screens.
Flat panel field emission displays (FEDs), like standard cathode ray tube (CRT) displays, generate light by impinging high energy electrons on a picture element (pixel) of a phosphor screen. The excited phosphor then converts the electron energy into visible light. However, unlike conventional CRT displays which use a single or in some cases three electron beams to scan across the phosphor screen in a raster pattern, FEDs use stationary electron beams for each color element of each pixel. This allows the distance from the electron source to the screen to be very small compared to the distance required for the scanning electron beams of the conventional CRTs. In addition, FEDs consume far less power than CRTs. These factors make FEDs ideal for portable electronic products such as laptop computers, pagers, cell phones, pocket-TVs, personal digital assistants, and portable electronic games.
One problem associated with the FEDs is that the FED vacuum tubes may contain minute amounts of contaminants which can become attached to the surfaces of the electron-emissive elements, faceplates, gate electrodes, focus electrodes, (including dielectric layer and metal layer) and spacer walls. These contaminants may be knocked off when bombarded by electrons of sufficient energy. Thus, when an FED is switched on or switched off, there is a high probability that these contaminants may form small zones of high pressure within the FED vacuum tube.
Within an FED, electrons may also hit spacer walls and focus electrodes, causing non-uniform emitter degradation. Problems occur when electrons hit any surface except the anode, as these other surfaces are likely to be contaminated and out gas.
The problems associated with contaminants, electron bombardment and out gassing can lead to brightness variations from row-to-row in an FED device. These brightness variations can be most pronounced around the rows that are nearby spacer walls. Spacer walls are placed between the anode and emitters of an FED device and help maintain structural integrity under the vacuum pressure of the tube. One cause of brightness variations of rows nearby spacer walls results from a non-uniform amount of contaminants falling onto the emitters that are located near spacer walls. More contaminants falling on these emitters makes rows dimmer or brighter that are located nearby the spacer walls.
Another factor leading to brightness variations row-to-row is that electrons may strike the spacer walls thereby causing ions to be released which migrate to the emitters. These ions may make the rows closer to the spacer walls actually get brighter. Also, over the life of the tube, gasses exit the faceplate and the existence of the spacer walls causes a reduced amount of these gasses to be absorbed by the emitters near the spacer walls compared to those emitters that are located farther away from the spacer walls. As a result, the cathodes of the emitters located near the spacer walls are left in relatively good condition thereby leading to brighter rows near the spacer walls.
Unfortunately, the human eye is very sensitive to brightness variations of rows that are close together. These variations can cause visible artifacts in the display screen that degrade image quality.
It would be advantageous to reduce or eliminate brightness variations of the rows of an FED device. More specifically, it would be advantageous to reduce or eliminate brightness variations for rows located nearby spacer walls.
Accordingly, the embodiments of the present invention reduce or eliminate brightness variations of the rows of an FED device. More specifically, embodiments of the present invention reduce or eliminate brightness variations for rows located nearby spacer walls. Also, embodiments of the present invention provide an accurate method of measuring brightness variations of an FED device row-to-row. These and other advantages of the present invention not specifically described above will become clear within discussions of the present invention herein.
Methods are described for compensating for brightness variations in a field emission device. In one embodiment, a method and system are described for measuring the relative brightness of rows of a field emission display (FED) device, storing information representing the measured brightness into a correction table and using the correction table to provide uniform row brightness in the display by adjusting row voltages and/or row on-time periods. A special measurement process is described for providing accurate current measurements on the rows. This embodiment compensates for brightness variations of the rows, e.g., for rows near the spacer walls. In another embodiment, a periodic signal, e.g., a high frequency noise signal is added to the row on-time pulse in order to camouflage brightness variations in the rows near the spacer walls. In another embodiment, the area under the row on-time pulse is adjusted using a number of different pulses shaping techniques to provide row-by-row brightness compensation based on correction values stored in a memory resident correction table. In another embodiment, the brightness of each row is measured and compiled into a data profile for the FED. The data profile is used to control cathode burn-in processes so that brightness variations are corrected by physically altering the characteristics of the rows.
More specifically, in a field emission display (FED) device comprising: rows and columns of emitters; an anode electrode; and spacer walls disposed between the anode electrode and the emitters, one embodiment of the present invention is directed to a method of measuring display attributes of the FED device comprising the steps of: a) in a progressive scan fashion, sequentially driving each row and measuring the current drawn by each row, wherein a settling time is allowed after each row is driven; b) measuring a background current level during a vertical blanking interval; c) correcting current measurements taken during the step a) by the background current level to yield corrected current measurements; d) averaging multiple corrected current measurements taken over multiple display frames to produce averaged corrected current values for all rows of the FED device; and e) generating a memory resident correction table based on the averaged corrected current values.
In a field emission display (FED) device comprising: rows and columns of emitters; an anode electrode; and spacer walls disposed between the anode electrode and the emitters, another embodiment of the present invention includes a method of driving the FED device comprising the steps of: a) generating a correction signal that is periodic in nature; b) adding the correction signal to a row driving pulse to generate a corrected row driving pulse; c) using the corrected row driving pulse to drive a row of the rows for a row on-time period; and d) generating a display frame by repeating steps a)-c) for each of the rows and wherein the correction signal functions to camouflage any non-uniformities of display brightness associated with rows that are positioned near the spacer walls.
In a field emission display (FED) device comprising: rows and columns of emitters; an anode electrode; and spacer walls disposed between the anode electrode and the emitters, another embodiment of the present invention includes a method of driving the FED device comprising the steps of: a) accessing a memory resident correction table to obtain a row correction value for a given row, the correction table containing a respective correction value for each of the rows, the correction values used to adjust the brightness of the rows on a row-by-row basis to correct for any brightness non-uniformities of the rows; b) applying the correction value, of the given row, to a row on-time pulse to generate a corrected row on-time pulse; c) driving the given row with the corrected row on-time pulse; and d) displaying a frame by repeating the steps a) and c) for each of the rows.
Another embodiment of the present invention includes a field emission display (FED) device comprising: rows and columns of emitters; an anode electrode; spacer walls disposed between the anode electrode and the emitters, a memory resident correction table for supplying a respective correction value for each of the rows, the memory resident correction table for providing row-by-row brightness correction to compensate for row brightness variations near the spacer walls; a correction circuit coupled to the memory resident correction table and for applying correction values from the correction table to row on-time pulses to generate corrected row on-time pulses; and driver circuitry coupled to the correction circuit for driving the rows with the corrected row on-time pulses.
Another embodiment of the present invention is directed at a method of compensating for brightness variations within a field emission display (FED) device comprising: rows and columns of emitters; an anode electrode; and spacer walls disposed between the anode electrode and the emitters, the method comprising the steps of: a) generating a data profile for the FED by measuring the brightness of each row of the rows and storing therein a respective value for each row; and b) based on the data profile, performing a cathode burn-in process that alters the physical characteristics of the rows to compensate for brightness variations depicted in the data profile.